According to conventional technology, MEMS sensors are mounted in a package onto the surface of a printed circuit board or a flexible film. This makes the package thicker at the location of the sensor. In some applications such as flow sensors or pressure sensors in flowing media, it is important for the surface to be smooth and planar in order to not generate whirls and turbulences and to not disturb a laminar flow.
Until now, MEMS sensors were sold packaged in a housing and were mounted on printed circuit boards with these housings. Even small SMD (Surface Mount Device) housings have a thickness in the order of millimeters.
ASICs, controllers and many other circuits made of silicon are also mounted and electrically connected as a bare die (=silicon chips without a housing) on the inside of a printed circuit board. However, there are enclosed on all sides by the printed circuit board and the adhesives in order to be protected from environmental influences such as moisture and corrosion. However, with MEMS sensors such as pressure sensors, a contact is needed at least between the sensitive surface of the silicon chip and the environment. This also applies to other chemical sensors (pH-value, gases and ions in liquids, salt content) and physical sensors (temperature, pressure, sound, radiation, acceleration) if a thin covering layer effects the sensitivity of the sensor as well.